It has been suggested that megadont mandibular premolars in fossil hominins is an adaptation for the consumption of mechanically challenging food items. This study used an extant primate model to test the hypothesis that megadont premolars correlate with diets that are especially hard or tough. We investigated several mixed-sex groups of closely-related, sympatric primates (e.g., Callimico-Saguinus, Cebussp., Cercocebus-Lophocebus, Hylobates-Pongo, Macaca sp., Papio-Theropithecus). Although there is significant overlap in the dietary items these sympatric primates consume, they differ in their mandibular postcanine morphology. Standard metric and 2D geometric morphometric analyses were conducted on the mandibular postcanines of these taxa in the collection of the NMNH. Even when the effects of differences in body size are taken into account, relative premolar size is significantly and consistently larger in taxa that consume harder diets than closely-related, sympatric taxa. Relative premolar size was not consistently larger in taxa that consume tougher diets than closely-related, sympatric taxa. These results indicate that relatively large P4 crown areas may be an adaptation that allows taxa to shift to diets with higher percentages of mechanically challenging food items, specifically hard food items. The results of this study have implications for reconstructing the dietary ecology of sympatric Paranthropus and early Homo and may be evidence of character displacement in the evolution of the postcanines of early hominins.